Andre Agassi awoke yesterday morning in his Manhattan hotel room to horrible back pain. And for the first time in more than 20 years, it didn’t matter.

“He’s really feeling it,” his brother, Phillip Agassi, told The Post.

And that was the beauty of Agassi’s first full day of retirement. When asked Sunday how retirement’s first day would feel, to wake up like a normal person without worries about his physical condition, Agassi, 36, relished the thought.

“It’s like you’re talking dirty tome now. I like it,” he said. “I hope it feels really, really strange because that would reflect a lot of assumed responsibility, pressure and commitment no longer on my shoulders. I’m going to wake up and start with not caring how I felt.

That’s going to feel great.” After his loss Sunday to Benjamin Becker ended his 21-year career, Agassi, his entourage and their families celebrated over dinner at the East Side steakhouse, BLT Prime.

“It was a good celebration,” Phillip Agassi said.

“Friends and family talking about old times, new times to come. It was pretty cool.” Agassi’s legendary career ended amidst tears and a series of standing ovations after the four-set loss to Becker. The hobbled Las Vegas legend received a four-minute standing ovation after the match. Agassi looked as if he was going to burst into tears on match point, but broke down moments later, first in a towel, then for all the world to see.

Simultaneously, at Armstrong Stadium, play stopped as fans gave Agassi a two-minute standing ovation during the Marat SafinDavid Nalbandian match. Agassi received a standing ovation in the player’s locker room and again in the media interview room following a 35-minute question-andanswer before 150 journalists.

“Have you ever seen an athlete get a standing ovation in the interview room?” Philip asked. “It was very respectful of everybody. And it really touched him.” Always a showman, Agassi’s farewell tournament played out perfectly. It featured classic wins over Andrei Pavel and Marcos Baghdatis, complete with the drama of post-match injections for his back. Then came the final confirmation that his chronic back can no longer get through a Grand Slam event, even with the needles.

“He gets to go home,” longtime trainer Gil Reyes said. “He felt he owed the people, the fans, something for all they have given him.

The debt has been settled now.” The next big event in Agassi’s life is Oct. 7 at the MGM Grand when he holds his 11th annual fundraiser for the Andre Agassi College Preparatory Academy, a school for at-risk children in Las Vegas.

“That night is a huge night,” Agassi said. “That’s a perfect example of what I look forward to. In years past, I’m not playing in Europe because I’m doing this for the foundation.” Agassi finished with a 870-274 record, eight Slam titles and countless memories.

“He’s earned his place in history in the sport,” Reyes said. “That’s something that he understands because the history is so important to him.”